Class 10 Science: CBSE Sample Question Paper (2020-21) - 1

OR

Which of the following represents voltage?

(a) 

Q.17. The change in magnetic field lines in a coil is the cause of induced electric current in it. Name the underlying phenomenon.    (1 Mark)
Ans. Electromagnetic induction - an emf (and hence current) is induced in a coil when the magnetic flux through it changes with time.
Q.18. Answer question numbers 18(a) to 18(d) on the basis of your understanding of the following paragraphs and the related studied concepts.
The growing size of the human population is a cause of concern for all people. The rate of birth and death in a given population will determine its size. Reproduction is the process by which organisms increase their population. The process of sexual maturation for reproduction is gradual and takes place while general body growth is still going on. Some degree of sexual maturation does not necessarily mean that the mind or body is ready for sexual acts or for having and bringing up children. Various contraceptive devices are being used by human beings to control the size of the population.
(a) List two common signs of sexual maturation in boys and girls.    (1 Mark)
Ans. In boys: broadening of shoulders and appearance of facial hair. In girls: development of breasts (mammary glands) and the start of menstruation.
(b) What is the result of reckless female foeticide?    (1 Mark)
Ans. It causes a shortage of females in the population, creating a serious imbalance in the male-to-female ratio and leading to social problems.
(c) Which contraceptive method changes the hormonal balance of the body?    (1 Mark)
Ans. Chemical method of contraception such as oral contraceptive pills (hormonal pills).
(d) Write two factors that determine the size of a population.    (1 Mark)
Ans. Birth rate and death rate.
Q.19. Veins are thin-walled and have valves. Justify.    (1 Mark)
Ans. Veins carry blood at low pressure, so their walls are thinner than arteries. Valves in veins prevent backflow and ensure blood moves in one direction towards the heart, especially against gravity in the limbs.
Q.20. State the role of the pancreas in the digestion of food.    (1 Mark)
Ans. The pancreas secretes pancreatic juice containing enzymes such as amylase (for starch), trypsin (for proteins) and lipase (for fats), and also bicarbonate ions to neutralise acidic chyme entering the small intestine.

Section  B

Q.21. Tooth enamel is one of the hardest substances in our body. How does it undergo damage due to eating chocolates and sweets? What should we do to prevent it?    (2 Mark)
Ans. When sugars from chocolates and sweets remain in the mouth, oral bacteria ferment them to produce acids. If the mouth pH falls below about 5.5, these acids dissolve (demineralise) enamel and cause decay. To prevent this: brush teeth regularly with fluoride toothpaste, reduce sugary snacks between meals, rinse the mouth with water after sweets, and visit the dentist for regular check-ups.
Q.22. State the meaning of strong acids and weak acids. Give one example of each.   (2 Mark)
Ans. Strong acid: An acid that ionises (dissociates) almost completely in water to give many H⁺ ions. Example: Hydrochloric acid (HCl).
Weak acid: An acid that ionises only partially in water, producing relatively few H⁺ ions. Example: Ethanoic acid (CH₃COOH).
Q.23. Give the respective scientific terms used for studying : 
(i) the mechanism by which variations are created and inherited and, 
(ii) the development of a new type of organisms from the existing ones.    (2 Mark)
Ans. (i) Genetics - the study of how traits and variations are inherited.
(ii) Evolution - the study of how new types of organisms develop from existing ones over time.
Q.24. On entering in a medium from the air, the speed of light becomes half of its value in air. Find the refractive index of that medium with respect to air.    (2 Mark)
Ans. Let v_air be the speed of light in air and v_medium be the speed in the medium. Given v_medium = (1/2) v_air.
Refractive index µ (with respect to air) = v_air / v_medium = v_air / (v_air / 2) = 2.
Thus the refractive index of the medium with respect to air is 2.
Q.25. List four methods of contraception used by humans?    (2 Mark)
Ans. Four commonly used methods are:
(i) Mechanical barrier methods (e.g., condoms).
(ii) Chemical methods (e.g., spermicidal creams or foams).
(iii) Hormonal methods (e.g., oral contraceptive pills).
(iv) Surgical methods (e.g., tubectomy or vasectomy for permanent sterilisation).

OR
Justify the following statement: "The use of contraceptive methods has a direct effect on the health and prosperity of a family."
Ans. 

• Pregnancy places significant physical and financial demands on a woman and her family; if pregnancies occur too frequently they can harm the mother's health.
• Contraception allows spacing of children and family planning, improving maternal health, allowing better care and resources per child, and contributing to the overall prosperity of the family.

Q.26. "The number of trophic levels in a food chain is limited." Give a reason to justify this statement.    (2 Mark)
Ans. 

• Only about 10% of the energy at one trophic level is transferred to the next level (the rest is lost as heat, waste and used in life processes). This is known as the 10% law of energy transfer.
• Because energy decreases rapidly up the chain, after about 4-5 trophic levels there is too little energy left to support further levels, so the number of trophic levels is limited.

OR

We do not clean natural ponds or lakes but an aquarium needs to be cleaned regularly. Why is it so?
Ans. We need to clean the aquarium because of the :
(i) absence of sufficient natural decomposers and ecological cycles in a closed artificial tank.
(ii) stagnancy of water and accumulation of waste, which would otherwise be broken down in a natural ecosystem.

Section C

Q.27. Identify the acid and the base from which sodium chloride is obtained. Which type of salt is it? When is it called rock salt? How is rock salt formed?    (3 Mark)
Ans. Acid - Hydrochloric acid (HCl).
Base - Sodium hydroxide (NaOH).
Type of salt - Neutral salt (formed by neutralisation of an acid by a base).
It is called rock salt (halite) when it occurs as large crystalline deposits often mixed with impurities and forming brownish crystals.
Rock salt is formed by the evaporation and drying up of seas or saline lakes, leaving behind crystalline deposits of sodium chloride.

OR

(a) For the preparation of cakes, baking powder is used. If at home your mother uses baking soda instead of baking powder, how will it affect the taste of the cake and why?
(b) How is baking soda converted into baking powder?
(c) What makes the cake soft and spongy?
Ans.
(a) If baking soda (sodium bicarbonate) is used without an acid present, it can decompose on heating to form sodium carbonate (Na₂CO₃), giving a bitter taste to the cake.
(b) Baking powder is made by mixing baking soda with a mild edible acid (e.g., tartaric acid) and a drying agent; on heating the acid and base react to release CO₂.
(c) The CO₂ gas liberated during baking forms bubbles in the batter, making the cake soft and spongy.
Q.28. (i) Name the element with atomic number 17?    (3 Mark)
(ii) To which period does it belong?
(iii)To which group does it belong?
(iv) Write its electronic configuration.
Ans. (i) Chlorine.
(ii) 3rd period.
(iii) 17th group (halogens).
(iv) 2, 8, 7.
Q.29. Three resistors of 5 Ω, 10 Ω and 15 Ω are connected in series and the combination is connected to the battery of 30 V. Ammeter and voltmeter are connected in the circuit. Draw a circuit diagram to connect all the devices in the proper correct order. What are the current flowing and potential difference across 10 Ω resistance?    (3 Mark)
Ans.

Q.30. (a) Budding, fragmentation and regeneration, all are considered asexual mode of reproduction. Why?    (3 Mark)
(b) With the help of neat diagrams, explain the process of regeneration in Planaria?
Ans. (a) These are asexual because they involve only one parent, do not require fusion of gametes, and produce offspring genetically identical to the parent through mitotic cell divisions.
(b) In Planaria regeneration, if the body is cut into pieces, each piece can regrow the missing parts and develop into a complete worm. Specialised cells divide and differentiate to replace lost structures, so one original organism can give rise to several complete organisms by regeneration.

OR

Mention the total number of chromosomes along with the sex chromosomes that are present in a human female and a human male? Explain how in sexually producing organisms the number of chromosomes in the progeny remains the same as that of the parents?
Ans. Chromosome numbers:
Human somatic cells have 46 chromosomes in total (23 pairs). Of these, 22 pairs are autosomes and 1 pair is the sex chromosomes.
In a human male: 22 pairs of autosomes + XY sex chromosomes (total 46).
In a human female: 22 pairs of autosomes + XX sex chromosomes (total 46).
How the number is maintained:
During gamete formation (meiosis), the chromosome number is halved so gametes are haploid (23 chromosomes). When a male gamete (sperm) and a female gamete (egg) fuse at fertilisation, the diploid number (46) is restored in the zygote. This keeps the chromosome number constant across generations.
Detailed Answer

• The total diploid number in humans is 46 chromosomes (23 pairs).
• Male gametes and female gametes are haploid (23 chromosomes). Fusion of two haploid gametes forms a diploid zygote with 46 chromosomes, ensuring equal genetic contribution from both parents.

Q.31. In a pea plant, the trait of flowers bearing purple colour (PP) is dominant over white colour (pp). Explain the inheritance pattern of F₁ and F₂ generations with the help of a cross following the rules of inheritance of traits. State the visible characters of F₁ and F₂ progenies?    (3 Mark)
Ans. Cross: Pure-breeding purple (PP) × pure-breeding white (pp):

Q.34. Carbon cannot reduce the oxides of sodium, magnesium and aluminium to their respective metals. Why? Where are these metals placed in the reactivity series? How are these metals obtained from their ores? Take an example to explain the process of extraction along with chemical equations.    (5 Mark) 
Ans. 

• These metals (sodium, magnesium, aluminium) are high in the reactivity series and have a stronger affinity for oxygen than carbon. Because their oxides are more stable, carbon cannot reduce these oxides to the metal.
• They are placed above carbon in the reactivity series (i.e., more reactive than carbon).
• Such metals are usually obtained by electrolytic reduction of their molten compounds rather than by reduction with carbon.
  For example, extraction of aluminium from its ore (bauxite) involves purification to obtain alumina (Al₂O₃) followed by electrolysis of molten alumina (Hall-Héroult process):
  At cathode: Al³⁺ + 3e^- → Al (aluminium metal)
  At anode: 2O^2- → O₂ + 4e^-
  This electrolytic process yields pure aluminium metal and oxygen gas.

Q.35. The position of certain elements in the Modern Periodic Table is shown below.

Using the above table answer the following questions giving reasons in each case:    (5 Mark) 
(i) Which element will form only covalent compounds?
(ii) Which element is a non-metal with valency 2?
(iii) Which element is a metal with valency 2?
(iv) Out of H, C and F which has the largest atomic size.
(v) To which family does H, C and F belong.
Ans. (i) Element E is silicon. It will form covalent bonds because it has four electrons in its outermost shell and typically shares electrons to complete its octet.
(ii) Non-metal with valency 2 is B, which is oxygen.
(iii) Element D is a metal with valency 2 (magnesium). Its low electronegativity makes it likely to lose two electrons to form Mg²⁺.
(iv) Element F has the largest atomic size. Element F is argon; it occupies more principal energy levels than H and C, so its atomic radius is the largest among the three.
(v) Elements H, C and F belong to Group number 18 in the given table (noble gases) as shown; these elements have their outer shells complete and are chemically stable, placing them in the noble gas family according to the table provided.

OR

Define atomic size. Give its unit of measurement. In the modern periodic table what trend is observed in the atomic radius in a group and a period and why is it so?
Ans. Atomic size (atomic radius) is the distance from the centre of the nucleus to the outermost electron shell of an atom.
Unit: Angstrom (Å), where 1 Å = 10^-10 m.
Trend across a period (left to right): Atomic radius decreases because nuclear charge increases and pulls electrons closer, reducing size.
Trend down a group: Atomic radius increases because electrons occupy higher principal energy levels (more shells), making atoms larger despite increased nuclear charge.
Reasons:
Across a period, increased effective nuclear charge draws electrons inward. Down a group, added shells outweigh increased nuclear attraction, increasing size.
Q.36. (a) Why is there a difference in the rate of breathing between aquatic organisms and terrestrial organisms? Explain.
(b) Draw a diagram of the human respiratory system and label - pharynx, trachea, lungs, diaphragm and alveolar sac on it.     (5 Mark) 
Ans. (a) Terrestrial organisms breathe air, which contains a high concentration of dissolved oxygen and is easy to obtain. Aquatic organisms rely on the small amount of oxygen dissolved in water, which is much less available than in air. Therefore aquatic organisms often have a higher rate of breathing or specialised structures (gills) to extract enough oxygen from water.
(b) Diagram of the human respiratory system :

OR